Microelectromechanical systems (MEMS) are small integrated devices or systems that combine electrical and mechanical components. The components can range in size from the sub-micrometer level to the millimeter level, and there can be any number, from one, to few, to potentially thousands or millions, in a particular system. Historically MEMS devices have leveraged and extended the fabrication techniques developed for the silicon integrated circuit industry, namely lithography, doping, deposition, etching, etc. to add mechanical elements such as beams, gears, diaphragms, and springs to silicon circuits either as discrete devices or in combination with integrated silicon electronics. Whilst the majority of development work has focused on silicon electronics additional benefits may be derived from integrating MEMS devices onto other existing electronics platforms such as silicon germanium (SiGe), gallium arsenide and, indium phosphide for RF circuits and future potential electronics platforms such as organic based electronics, nanocrystals, etc.
Examples of MEMS device application today include inkjet-printer cartridges, accelerometers, miniature robots, micro-engines, locks, inertial sensors, micro-drives, micro-mirrors, micro actuators, optical scanners, fluid pumps, transducers, chemical sensors, pressure sensors, and flow sensors. New applications are emerging as the existing technology is applied to the miniaturization and integration of conventional devices. These systems can sense, control, and activate mechanical processes on the micro scale, and function individually or in arrays to generate effects on the macro scale. The micro fabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks, or in combination can accomplish complicated functions.
The MEMS market is currently projected to exceed US $10 Billion in 2011, doubling from its estimated 2005 revenues of $5 Billion, according to Semiconductor Partners (Phoenix) (“Driving MEMS beyond Automotive” September 2007). Whilst historically the automotive MEMS market has dominated and will still show robust growth as the number of MEMS devices per vehicle increases from an average of 40 per mid-range vehicle to ˜60 MEMS for the same class of vehicle in 2011, it is the potential for growth in the consumer, communication and portable device markets that is more significant. Such applications include monolithic microphones and loudspeakers, oscillators, handheld controls for gaming devices and cellular telephones, hard disk drives, RF switches and ink-jet print heads. The MEMS mobile cellular telephone market alone is expected to exceed US $0.5 Billion in 2008 according to Research and Markets (“Mems4Mobile 06: Updated Analysis of the Applications and Markets of MEMS in Mobile Communications” February 2006). Such MEMS devices incorporated into cellular telephones potentially including silicon microphones, 3D accelerometers, gyroscopes for camera stabilization and GPS navigation, microfuel cells, personal weather stations, and biochips for health care monitoring.
MEMS have become a successful sensing and actuating technology. Because of their extensive optical, electrical to mechanical functionalities, MEMS devices are suited to applications in many different fields of science and engineering. However, because of this vast range of functionality, MEMS fabrication processes, unlike the microelectronics industry, are difficult to gear towards general applications. As a result most processes are aimed at the fabrication of a few devices, and usually performance of the devices is hindered by process variability. As MEMS devices are typically sensing weak analog signals, for example pressure, acceleration, vibration, magnetic or electric fields, with capacitive based elements, there is considerable benefit in being able to integrate analog front-end electronics to buffer, amplify and process these weak electronic signals and either facilitate their direct processing, such as with RF signals, or their digitization for sensing and measurements applications.
Historically CMOS electronics has become the predominant technology in analog and digital integrated circuits. This is essentially because of the unparalleled benefits available from CMOS in the areas of circuit size, operating speed, energy efficiency and manufacturing costs which continue to improve from the geometric downsizing that comes with every new generation of semiconductor manufacturing processes. In respect of MEMS systems, CMOS is particularly suited as CMOS circuits dissipate power predominantly during operation and have very low static power consumption. This power consumption arises from the charging and discharging of various load capacitances within the CMOS circuits, mostly gate and wire capacitance, but also transistor drain and transistor source capacitances, whenever they are switched. The charge moved is the capacitance multiplied by the voltage change. Multiply by the switching frequency to get the current used, and multiply by voltage again to get the characteristic switching power, P, dissipated by a CMOS device, and henceP=CV2f  (1).
Historically, CMOS designs operated at supply voltages (Vdd) much larger than their threshold voltages (Vth), for example Vdd=5V and Vth=700 mV for both NMOS and PMOS. However, today CMOS manufacturers have adjusted designs and materials such that today an NMOS transistor may have a Vth of 200 mV and allow operation from voltages as low as Vd=1V offering a significant power reduction which is important in sensing, mobile, chemical and biomedical applications.
However, combining CMOS and MEMS technologies has been especially challenging because some MEMS process steps—such as the use of special materials, the need for high temperature processing steps, the danger of contamination due to the MEMS wet etching processes etc.—are incompatible with the requirements of CMOS technology. Thus, strong attention has to be paid to avoid cross contaminations between both process families. Accordingly today MEMS processes exist that are discrete and standalone, such as Robert Bosch's (U.S. Pat. No. 5,937,275 “Method of Producing Acceleration Sensors”, MEMSCAP's “Multi-User MEMS Processes” (MUMPs® including PolyMUMPs™, a three-layer polysilicon surface micromachining process: MetalMUMPs™, an electroplated nickel process; and SOIMUMPs™, a silicon-n-insulator micromachining process), and Sandia's Ultra-planar Multi-level MEMS Technology 5 (SUMMiT V™ Fabrication Process which is a five-layer polycrystalline silicon surface micromachining process with one ground plane/electrical interconnect layer and four mechanical layers).
Other processes have been developed to allow MEMS to be fabricated before the CMOS electronics, such as Analog Devices' MOD-MEMS (monolithically integrate thick (5-10 um) multilayer polysilicon MEMS structures with sub-micron CMOS), and Sandia's iMEMS. Finally, processes have been developed to provide MEMS after CMOS fabrication such as Sandia's micromechanics-last MEMS, Berkeley Sensor & Actuator Center (BSAC), and IMEC silicon-germanium processes. Additionally DALSA Semiconductor have a highly publicized “low temperature” micro-machining with silicon dioxide process, see L. Ouellet et al (U.S. Pat. No. 7,160,752 “Fabrication of Advanced Silicon-Based MEMS Devices”, Issued Jan. 9, 2007) wherein low stress structures were fabricated at temperatures between 520° C. and 570° C., being just below the temperature of eutectic formation in aluminum-silicon-copper interconnections.
However, the mechanical properties of silicon do not make it the most suitable structural material for MEMS. Recently, silicon carbide (SiC) has generated much interest as a MEMS structural material because of its distinctive properties. SiC boasts better suited mechanical properties such as higher acoustic velocity, high fracture strength and desirable tribological properties. Its ability to sustain higher temperatures, and resist corrosive and erosive materials makes SiC, unlike silicon, a potential candidate material for use in harsh environments. SiC is also being investigated and shows promise as a biocompatible material, see for example “Porous Silicon Carbide as a Membrane for Implantable Biosensors” (A. J. Rosenbloom et al, Biomedical Microdevices, Vol. 6, No. 4, December 2004, Springer), “Biocompatibility of Silicon Carbide in Colony Formation Test in Vitro” (S. Santavirta et al, Archives of Orthopedic and Trauma Surgery, Vol. 118, Nos. 1-2, November 1998), and “SiC Based Artificial Dental Implant” (U.S. Pat. No. 5,062,798, K. Tsuge et al). These factors, along with the maturation of deposition and patterning techniques, make SiC a potential choice for high-performance MEMS processing.
However, difficulties with SiC processing have made its use non-trivial as it is non-conductive and difficult to deposit and dope at CMOS compatible temperatures. Stress control is also difficult because of the high intrinsic stresses that can develop in such a material. Because of its intrinsic inertness, selective etching of SiC is difficult. As most materials are etched at a faster rate than SiC, issues arise when masking SiC for patterning and ensuring a reliable etch-stop. Whether it is for doping or for deposition, SiC needs to generally be processed at high temperatures. As such prior art SiC MEMS processes have not lent themselves well to CMOS integration. Further as most MEMS applications require electrical signal processing, integration of MEMS to transistor-able processes, such as CMOS, is paramount.
It would be advantageous to overcome at least some of the disadvantages of the prior art.